Valves of this kind are very generally composed of a hollow body mounted on the vessel and communicating with the interior of the latter. Inside the hollow body a movable closure member mounted on the end of a rod is adapted to move between a closure position, in which it bears in a leaktight manner against its seat, and an open position in which it is moved away from the seat. A calibrated spring is in compression between a member fixed to the valve body and a member fixed to the closure member rod, in such a manner as to apply to the closure member a thrust holding it in the closed position as long as the pressure in the vessel does not exceed the preset value.
When the closure member is lifted through the action of an excessive pressure, part of the fluid under pressure is discharged from the vessel, thus making it possible to bring the pressure back to a value lower than the preset value, and the calibrated spring returns the closure member to its closed position on its seat.
However, numerous factors may disturb the operation of ordinary valves of this kind, such as, for example, the formation, at the moment of opening, of pressure waves in the fluid contained in the vessel, or, in the case of incompressible fluids, the formation of local negative pressures just upstream of the closure member, thus causing the latter to close before the pressure has actually fallen throughout the vessel. All these factors result in unstable operation of the valve and hunting of the closure member, which may in turn give rise to the seizure of the valve or hammering in the pipes protected by the valve.
In order to eliminate these disadvantages, it appears necessary to provide assistance for the operation of the closure member. Assistance in opening amounts to momentarily reducing the calibration of the spring in order to obtain clean, rapid opening when the pressure in the vessel attains the critical value, and preventing the closure member from tending to reclose immediately through the action of a local fall in pressure; on the other hand, assistance in closing amounts to increasing the calibration in order to ensure sharp closure and preventing inadvertent reopening when the total pressure in the vessel has dropped back below the preset value.
For this purpose various assistance devices have already been proposed which utilize magnetic or pneumatic means, but they always have, either separately or simultaneously, various kinds of disadvantages. In some cases, assistance both in opening and in closing is not achieved, or else these devices constitute only simple dampers for the movement of the closure member.
Other devices provide only assistance in locking at the end of the opening or closing movement of the closure member. Certain devices are not entirely autonomous and depend on a general compressed air distribution system or an electricity supply system, so that their operational reliability may be jeopardized in the event of a failure of the supply, although the device should then continue to serve its safety function. Still other devices make use of complex fluid distribution circuits, thereby increasing the risk of failure. Finally, it is advantageous in industry to be able to fit such assistance devices to conventional valves already installed, and also to be able to make tests without load, without having to dismantle the equipment; this last point is particularly important in pressure vessels in nuclear reactors.